System and process for high-density, low-energy plasma enhanced vapor phase epitaxy

ABSTRACT

An apparatus and process for fast epitaxial deposition of compound semiconductor layers includes a low-energy, high-density plasma generating apparatus for plasma enhanced vapor phase epitaxy. The process provides in one step, combining one or more metal vapors with gases of non-metallic elements in a deposition chamber. Then highly activating the gases in the presence of a dense, low-energy plasma. Concurrently reacting the metal vapor with the highly activated gases and depositing the reaction product on a heated substrate in communication with a support immersed in the plasma, to form a semiconductor layer on the substrate. The process is carbon-free and especially suited for epitaxial growth of nitride semiconductors at growth rates up to 10 nm/s and substrate temperatures below 1000° C. on large-area silicon substrates. The process requires neither carbon-containing gases nor gases releasing hydrogen, and in the absence of toxic carrier or reagent gases, is environment friendly.

RELATED APPLICATIONS

This application is a US National Stage of PCT application claimingpriority to U.S. Provisional Application Ser. No. 60/657,208, filed 28Feb. 2005, the content of which is incorporated by reference.

FIELD OF THE INVENTION

This invention is in the field of epitaxy growth processes and coatingapparatuses. More specifically, the present invention relates toapparatuses and processes for epitaxy forming of a single crystal bydeposition of material directly from the vapor or gaseous state.

BACKGROUND OF THE INVENTION

The III-V compound semiconductor Gallium Nitride (GaN) and its Aluminum(Al) and Indium (In) alloys are ideal materials both for high-frequencyand high-power electronic applications (see for example Brown et al.,Solid-State El. 46, 1535 (2002), the content of which is incorporatedherein by reference thereto). These materials are also ideal for shortwavelength light emitting diodes and lasers (see for example Nakamura,Annu. Rev. Mater. Sci. 28, 125 (1998); Nakamura, Science 281, 956(1998); and Smith et al., J. Appl. Phys. 95, 8247 (2004), the contentsof which are incorporated herein by reference thereto).

One of the main drawbacks of the material is, however, the lack oflarge, single crystals due to extreme conditions of high temperature andpressure required for their growth in bulk form. The only way tosynthesize GaN wafers of significant size is by means of heteroepitaxy,whereby thick, self-supporting GaN layers are grown onto a supportsubstrate such as sapphire or Silicon Carbide (SiC), which substrate issubsequently removed. Thinner heteroepitaxial III-V nitride layers canbe used for device processing without removal of the substrate.

One common problem of all techniques used for heteroepitaxial growth ofGaN is the high dislocation density initially present in the growinglayers. This problem results from the different lattice parameters ofGaN and the available substrate materials, such as sapphire, siliconcarbide and silicon (see for example Dadgar et al., Phys. Stat. Sol. (c)0, 1583 (2003), the content of which is incorporated by referencethereto). As a consequence of the high misfit dislocation density,heteroepitaxial GaN layers tend to contain also high density ofthreading dislocations (TD), which degrade device performance wheneverthey penetrate into any active layers. Many ways have been devised toreduce TD densities to values acceptable for device fabrication, such asbuffer layer growth of various forms, or lateral overgrowth with andwithout the use of masks (see for example Davis et al., Proc. IEEE 90,993 (2002), the content of which is incorporated by reference thereto).

The main methods used for growing epitaxial III-V nitride layers arehydride vapor phase epitaxy (HVPE), metal-organic chemical vapordeposition (MOCVD) and molecular beam epitaxy (MBE). In HVPE, puremetals are used as source materials, and transported as gaseous halidesto the reaction zone where they react with a nitrogen-containing gas,usually NH₃ to form an epitaxial layer on a substrate typically heatedto above 1000° C. HVPE has the advantage of very high growth rates of upto 100 μm/h (see for example U.S. Pat. No. 6,472,300 to Nikolaev et al.,the content of which is incorporated herein by reference). Because ofits high growth rates, HVPE is mostly used for growing layers many tensof microns thick, and in particular for the fabrication ofself-supporting layers as substrates for subsequent MOCVD or MBE steps.

Low rates and control of sharp interfaces are, however, more difficultto achieve by HVPE, and may require mechanical movement of the substratebetween different reactor zones (see for example U.S. Pat. No. 6,706,119to Tsvetkov et al., the content of which is incorporated herein byreference). Additionally, the presence of hydrogen gas in the reactionzone requires annealing of the substrate in an inert gas atmosphere,particularly when high p-type doping for example by Mg impurities is tobe attained (see for example U.S. Pat. No. 6,472,300 to Nikolaev et al.,the content of which is incorporated herein by reference).

MOCVD (or MOVPE, for “metal-organic vapor phase epitaxy”) is a CVDtechnique in which metal-organic precursors are used along with otherreactive gases containing the anions, such as ammonia in the case ofnitride growth. The need for expensive precursor gases, along withrather low growth rates of just a few units, is a significantdisadvantage of MOCVD. Furthermore, a buffer layer usually has to begrown for GaN heteroepitaxy on sapphire, SiC or Si, at a lower substratetemperature before the active layer stacks are deposited at temperaturesabove 1000° C. (see for example U.S. Pat. No. 6,818,061 to Peczalski etal. the content of which is incorporated herein by reference). MOCVD is,however, the technique most often used for growing active layerstructures suitable for device fabrication (see for example Wang et al.,Appl. Phys. Lett. 74, 3531 (1999) and Nakamura, Science 281, 956 (1998)the contents of which are incorporated herein by reference).

Large differences in thermal expansion coefficients between commonsubstrates and GaN, together with the high substrate temperature duringgrowth, present a significant obstacle towards achieving crack-freeepitaxial layers. Crack avoidance seems to necessitate rathercomplicated interlayer schemes (see for example Bläsing et al., Appl.Phys. Lett. 81, 2722 (2002) the content of which is incorporated hereinby reference). HVPE and MOCVD are both deposition techniques working atatmospheric or somewhat reduced pressures. Reactor geometries and gasflows determine layer uniformities to a large extent.

By contrast, in MBE, pressures are in the high-vacuum toultrahigh-vacuum range and mean-free paths therefore greatly exceedreactor dimensions. Metals are evaporated in so-called effusion cellsfrom which molecular or atomic beams travel towards the heated substratewithout being scattered in the gas phase. For nitride growth, a nitrogensource yielding activated nitrogen must be used. Activation is usuallyachieved by means of plasma excitation of molecular nitrogen. A systemfor epitaxially growing Gallium Nitride layers with a remoteelectron-cyclotron-resonance (ECR) plasma source for nitrogen activationhas been described for example in U.S. Pat. No. 5,633,192 to Moustakaset al., the content of which is incorporated herein by reference. SinceGallium (Ga) is usually supplied from an effusion cell, MBE does notrequire the expensive metal-organic precursors common in MOCVD. MBEoffers moreover excellent control over layer composition and interfaceabruptness (see for example Elsass et al, Jpn. J. Appl. Phys. 39, L1023(2000); the content of which is incorporated herein by reference). Dueto its low growth rates on the order of 1 μm/h and complex equipment,however, it is not considered to be a technique suitable for large scaleproduction of semiconductor heterostructures.

A further method potentially suitable for large scale production ofnitride semiconductors (see, for example, U.S. Pat. No. 6,454,855 to vonKänel et al. the contents of which is incorporated herein by reference)is low-energy plasma enhanced chemical vapor deposition (LEPECVD). Incontrast to plasma assisted MBE, where nitrogen activation occurs in aremote plasma source, a dense low-energy plasma is in direct contactwith the substrate surface in LEPECVD. The low-energy plasma isgenerated by a DC arc discharge by means of which metalorganicprecursors and nitrogen are activated (see, for example, U.S. Pat. No.6,918,352 to von Känel et al. the content of which is incorporatedherein by reference). Potentially, LEPECVD can reach growth ratescomparable to those of HVPE (several tens of μm/h, while offeringoptimum control over the dynamic range of growth rates, such thatexcellent interface quality can be achieved. Moreover, since activationof the reactive precursors is achieved by means of a plasma rather thanthermally, the process is expected to work at lower substratetemperatures. The DC plasma source used in LEPECVD has been shown to bescalable to 300 mm substrates (see for example WO 2006/000846 to vonKänel et al., the content of which is incorporated herein by reference).

Although the term “LEPECVD” has been coined in conjunction with a DC arcdischarge (see, Rosenblad et al., J. Vac. Sci. Technol. A 16, 2785(1998), the content of which is incorporated herein by reference), sucha DC arc discharge is not the only way to generate a low-energy plasmasuitable for epitaxy. According to prior art, sufficiently low-energyions suitable for epitaxial growth may result also fromelectron-cyclotron-resonance (ECR) plasma sources (see Heung-Sik Tae etal., Appl. Phys. Lett. 64, 1021 (1994), the content of which isincorporated herein by reference). An ECR plasma source potentiallysuitable for epitaxial growth by plasma enhanced CVD on large areasubstrates has been described, for example, in U.S. Pat. No. 5,580,420to Katsuya Watanabe et al., the content of which is incorporated hereinby reference. In industrial semiconductor processing, large ECR sourcesare, however, used for etching rather than epitaxy. Very high etch rateshave been achieved in the case of III-V nitrides (see for example,Vartuli et al., Appl. Phys. Lett. 69, 1426 (1996), the content of whichis incorporated herein by reference).

Yet other sources of high-density, low-energy plasmas are inductivelycoupled plasma (ICP) sources. These sources have a number of advantagesover ECR sources, such as easier scaling to large wafer diameters andlower costs. For a review of the different kinds of ICP sources, seeHopwood, Plasma Sources Sci. Technol. 1, 109 (1992), the content ofwhich is incorporated herein by reference. The most common variants usedfor plasma processing are helical inductive couplers where a coil iswound around the plasma vessel (for example, see Steinberg et al., U.S.Pat. No. 4,368,092, the content of which is incorporated herein byreference), and spiral inductive couplers with flat coils in the form ofa spiral (for example, see U.S. Pat. No. 4,948,458 to Ogle, the contentof which is incorporated herein by reference). Plasma sources based onspiral couplers have the advantage of higher plasma uniformity,facilitating scaling to large substrate sizes (for example, see Collisonet al., J. Vac. Sci. Technol. A 16, 100 (1998), the content of which isincorporated herein by reference).

While ICP sources normally are operated at a frequency of 13.56 MHz,operating at lower frequency has been shown to decrease capacitivecoupling and thus leading to even lower ion energies (see U.S. Pat. No.5,783,101 to Ma et al., the content of which is incorporated herein byreference).

Both, ECR sources and ICP sources are usually used for etching. Veryhigh etch rates for GaN have been obtained also with ICP sources (seeShul et al., Appl. Phys. Lett. 69, 1119 (1996), the content of which isincorporated herein by reference). However, use of these sources forepitaxial growth of semiconductor quality materials is very rare.Recently, it has been suggested to apply an electrically-shielded ICPsource for ion plating epitaxial deposition of silicon. This method hasthe obvious drawback of requiring a metallic collimator inside thedeposition chamber (see U.S. Pat. No. 6,811,611 to Johnson, the contentof which is incorporated herein by reference).

ICP sources can also be used for efficient cleaning of process chambers,such as chambers used for thermal CVD, where a remote plasma source isusually employed (see U.S. Pat. No. 5,788,799 to Steger, the content ofwhich is incorporated herein by reference). Chamber cleaning isparticularly important for semiconductor processing, where particulatecontamination has to be kept as low as possible. Processing chambersequipped with a plasma source, such as an ICP source, do not of courserequire an additional remote source for efficient cleaning (see U.S.Pat. No. 6,992,011 to Nemoto et al., the content of which isincorporated herein by reference).

Whatever plasma source is used for generating a low-energy plasma forplasma enhanced chemical vapor deposition, when applied to III-Vcompound semiconductor growth, carbon incorporation into the growinglayers is likely to occur to a much greater extent than in MOCVD. Carbonincorporation results from the use of organic precursors in MOCVD and assuggested for LEPECVD (see U.S. Pat. No. 6,454,855 to von Känel et al.the content of which is incorporated herein by reference). The intenseplasma used for cracking the precursors in LEPECVD is expected togreatly enhance unintentional carbon uptake, possibly to a degreeunacceptable for device applications, since carbon acts as a dopant(see, for example, Green et al., J. Appl. Phys. 95, 8456 (2004), thecontent of which is incorporated herein by reference).

It is an objective of the present invention to avoid the drawbacks ofprior art techniques mentioned above, such as carbon and hydrogenincorporation, high substrate temperatures, and low deposition rates. Anadditional major limitation of prior art techniques is a relativelysmall wafer size (two inch in production, up to 6 inch demonstrated forsapphire substrates). Increased scaling of silicon wafers of up to 300mm (or more) is one of the objects of the present invention.

SUMMARY OF THE INVENTION

The present invention is a new low-energy, high density apparatus and aprocess for fast epitaxial deposition of compound semiconductor layerson to a semiconductor support substrate. The invention provides for thedeposition of a large variety of compound layers by being able tocontrollably alter constituent reagents and/or their concentrationsduring the deposition process. In a first step of the process, one orseveral metals are vaporized, and the metal vapors are injected into theinterior of the deposition chamber of the apparatus. Vaporization can beaccomplished using, for example, effusion cells or sputter targetscommunicating with the interior of the deposition chamber. Concurrently,upon injection of the metal vapor (e.g., Gallium) into the chamber, anon-metallic and normally non-reactive, non-toxic gas (Nitrogen as N₂)is also injected into the chamber. In a second substantially concurrentstep, a dense, low-energy plasma is generated and maintained in thedeposition chamber by any of a plurality of plasma generating mechanisms(such as an electron-cyclotron-resonance (ECR) plasma, an inductivelycoupled plasma (ICP) or a DC arc discharge plasma). When fully immersedin the plasma, the non-metallic gas becomes highly activated, and reactswith the metal vapor and forms an epitaxial semiconductor layer (e.g.,GaN) on a heated semiconductor substrate supported in the plasma. Theinvention provides a carbon-free process, because of the absence oforganic precursor reagents, and is especially well suited forapplication to producing semiconductor layers on large-area siliconsubstrate. Additionally, in the absence of any toxic carrier or reagentgases, the process is also extraordinarily environment friendly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side-view drawing of the system for low-energyplasma-enhanced vapor phase epitaxy (LEPEVPE) with an inductivelycoupled plasma (ICP) source and effusion cells.

FIG. 2 is a schematic drawing of a growing film on a substrate exposedto a low-energy plasma.

FIG. 3 is a schematic view of a plasma confined by a magnetic field.

FIG. 4 is a schematic side-view drawing of a variant of a system forLEPEVPE with an ICP source and effusion cells, and with substrate facedown.

FIG. 5 is a schematic side-view drawing of a system for LEPEVPE with anICP source and sputter sources.

FIG. 6 is a schematic drawing representing a system of the presentinvention for LEPEVPE with a DC plasma source and effusion cells.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a system including an apparatus and process forthe epitaxial growth of III-V semiconductors, especially groupIII-nitrides, such as GaN, GaAlN, and GaInN. The apparatus provides alow-energy, high-density plasma for plasma enhanced vapor phase epitaxyof semiconductor layers on to a semiconductor support. The presentsystem allows for the economical fabrication of heterostructuressuitable for high-frequency power amplifiers, violet, blue and whiteLEDs (lighting), and blue and ultra-violet semiconductor lasers.

Referring now to FIG. 1, the apparatus 10 includes a vacuum depositionchamber 20 having a chamber interior 21 communicating with a vacuumpumping system (not shown), such as a turbomolecular pump, attached toexhaust line 24. The deposition chamber 20 and the pumping system arechosen such as to being compatible with ultra-clean processing ofsemiconductors. For example a system allowing for ultra-high vacuum inthe absence of process gases has been found to be adequate. Inert andnormally non-reactive gases, such as argon and nitrogen, and anyadditional gases suitable for processing, are supplied to the depositionchamber 20 by means of gas inlets 22. Nitrogen in the form of N₂ isnormally a non-reactive gas. However, when exposed to the field of thepresent apparatus, the N₂ nitrogen is converted to its atomic form N andbecomes highly activated and reactive. The deposition chamber 20 isequipped with a dielectric window 28 through which radio frequency wavesare coupled into the chamber interior 21 by means of a spiral coilassembly 30. The spiral coil assembly 30 communicates with an impedancematching network 32 and a radio frequency generator 34. Radio frequencywaves emanating from the spiral coils excite a dense, low-energy plasmawithin the interior 21 of the chamber 20. For example, the inductivelycoupled plasma source ICP-P 200 from JE PlasmaConsult, GmbH inWuppertal, Germany, has been shown to yield Argon and Nitrogen ionenergies below 20 eV when operated in the pressure range between 10⁻⁴and 10⁻² mbar and powers up to 1 kW.

A deposition assembly 50 is electrically insulated from depositionchamber 20 by means of insulators 26. One or more substrate supports 54are heated from the back by a heating means 52, such as a resistiveheater or by lamp heaters. The substrate support 54 is spaced severalskin depths (typically 5-20) away from the location of highest plasmadensity close to the dielectric window 28. The skin depth is on theorder of 1 cm for the typical operating pressures used according to theinvention. The deposition assembly 50 can either be grounded or leftelectrically floating. Alternatively, the assembly 50 can be connectedto a DC bias power supply, or it can be coupled through an impedancematching network 56 to RF generator 58, giving rise to a DC self-bias.These measures are taken in order to control the electrical potential ofsubstrates 54 with respect to that of the plasma. In this way theelectric field component perpendicular to the surface of substrates 54can be controlled independently from the parameters controlling theplasma 36. The energy of ions impinging on the substrates can thus beadjusted for optimum epitaxial growth conditions.

In addition, the deposition chamber 20 is equipped with one or moremetal vapor emitters 40 (effusion cells in the embodiment illustrated)from which metals, such as Ga, In and Al, can be vaporized and thevapors injected into the chamber interior 21. For these metals, thetemperature of standard effusion cells used in molecular beam epitaxy(MBE) can easily be adjusted such as to allow much higher evaporationrates than those customary in that technique. For example an increase ofGallium cell temperature by 200° C. was found to be adequate for a100-fold increase of the GaAs growth rate of 1 monolayer/sec typical inMBE. Similar to MBE, fast-action shutters 42 are controllable tointerrupted completely the fluxes from the vapor emitters 40.

During epitaxial deposition, the radio frequency power applied to theinduction coils 30 and the gas pressures in the chamber 20 are chosensuch that the heated substrates 54 are fully exposed to a low-energyplasma. Typically, gas pressures in chamber 20 range between 10⁻⁴ mbarto 1.0 mbar, with pressures in the range of 10⁻² to 10⁻¹ mbar being themost typical. Under such conditions, activated nitrogen and metal vaporfrom effusion cells 40 both move by diffusive transport in the plasma.Metal atoms reacting with the nitrogen form an epitaxial nitride layeron the hot substrates 54.

Referring now to FIG. 2, a detailed view of a growing film 55 on asubstrate 54 exposed to a low-energy plasma 36 can be seen. The iondensity in the plasma decreases exponentially from the dielectric window28 to the substrate 54. For example for the plasma source “ICP-P 200,”the ion density in a nitrogen plasma may still exceed 10¹¹ cm⁻³ at asubstrate located about 10 cm below dielectric window 28, when anitrogen pressure of 10⁻¹ mbar at a gas flow of 10 sccm, and a rf-powerof 1000 W are used. In order to keep the ion energy low, it may beadvantageous to keep the total gas pressure fixed, for example around10⁻¹ mbar, by admitting a controlled flow of Ar through gas inlets 22 toenter the vacuum chamber 20 along with nitrogen gas, when nitrogenpartial pressures substantially below 10⁻¹ mbar are used.

As a result of the efficient activation of the reacting species in adense plasma 36, and intense bombardment of the substrate surface 54 bylow-energy ions the substrate temperature can be significantly loweredwith respect to the substrate temperatures of 1000° C. and more typicalfor MOCVD. The problems of layer cracking due to different thermalexpansion coefficients of typical substrates (sapphire, silicon carbideand silicon) are hence expected to be greatly reduced.

Referring now to FIG. 3, a detailed view of part of the vacuum chamber20 is shown, where in order to confine the plasma 36, and to increaseits density and uniformity, the chamber is optionally equipped withcoils or permanent magnets 70. The magnetic field generated by thesecoils or permanent magnets helps in shaping the plasma. Even weak fieldsof the order of 10⁻³ to 10⁻² Tesla are considered to be sufficient tohave a beneficial effect.

In a preferred embodiment of the invention no reactive gases are usedfor epitaxial nitride semiconductor growth at all. Additional cells 40 amay contain those doping species which are preferably used in elementalform, such as Mg, Zn and similar metals acting as acceptor impurities.Similarly, dopants acting as donors, such as silicon, may be provided byadditional cells 40 a. These emitters 40 a (effusion cells) also areequipped with fast-acting shutters 42 permitting rapid and completeinterruption of the dopant vapors. The preferred substrate support 54choice is silicon in order to allow for scaling up to 300 mm wafers, andpotentially beyond. However, the use of other substrates employed instate of the art techniques is equally possible in the new techniqueaccording to the invention.

The combination of effusion cells for metal evaporation with a denselow-energy plasma suitable for epitaxial layer deposition has not beenproposed before. We call the new process low-energy plasma enhancedvapor deposition (LEPEVPE). LEPEVPE is a process being operated undercompletely different conditions with respect to all other knownprocesses, including LEPECVD where a DC plasma discharge and reactivegas phase precursors are used.

In one embodiment of the invention, the region of the vapor emitters 300is differentially pumped (320 in FIG. 6) in order to exclude thermalreactions with the hot metals inside and diffusive transport in theconnecting tube to the deposition chamber. In a preferred embodiment ofthe invention more than one vapor emitter 40 & 40 a (effusion cell) isused per evaporated metal. Each cell can be operated at a differenttemperature, thereby easily allowing rapid changes in growth rates ordoping densities by switching from one cell to another.

In another embodiment of the invention, additional gas lines 23 are usedto insert doping gases into the deposition chamber for those dopingelements which are preferably applied in gaseous form. The doping gases,such as Silane for n-type doping, are preferably diluted in anon-reactive gas, such as argon. The dynamic range of doping can beincreased by using more than one gas line per doping gas. In a preferredembodiment where vapor emitters 40 a of only the solid source type areused for doping, the process is operated hydrogen-free. This embodimentis especially desirable for p-doped GaN layers since a hydrogen-freeprocess does not need any dopant activation by thermal annealing. Theprocess of the invention is carbon-free because it does not require anycarbon-containing precursor gases.

In the preferred embodiment of the invention illustrated in FIG. 1, theassembly of substrate supports 54 is facing up. This configuration,customarily used in semiconductor processing, facilitates wafer handlingand design of the deposition assembly or substrate holder 50. Accordingto the invention, LEPEVPE is characterized by a high density low-energyplasma in direct contact with the surface of the substrate support 54.The surface of the substrate support 54 is therefore under intensebombardment of low-energy ions, the energy of which may be adjusted byappropriate choice of the substrate bias. This is in marked contrast toplasma processing methods using remote plasma sources, which typicallydeliver radicals only, whereas ion densities at the substrate surfaceare negligibly low. Heavy substrate bombardment by low-energy ions hasbeen shown to be beneficial to epitaxial growth of device qualitysemiconductor layers at extremely high growth rates of more than 5 nm/sat substrate temperatures as low as 500° C. (see, for example, von Känelet al., Appl. Phys. Lett. 80, 2922 (2002), the content of which isincorporated herein by reference). According to the invention very highthroughputs may therefore be expected by combining LEPEVPE with state ofthe art wafer handling tools (not shown).

According to the present invention, the apparatus 10 may be used forgrowing epitaxial III-V semiconductors, especially group III-nitridesonto specially treated single-crystal substrates 54. Possible surfacetreatments of substrates 54 may involve state of the art chemicalpre-cleans, in situ thermal cleans or plasma cleans, followed by in situformation of epitaxial templates, such as oxides, carbides orlow-temperature nitrides, suitable for subsequent epitaxial nitridesemiconductor growth.

Referring now to FIG. 4, an apparatus 10 of the resent system is shownin which the substrate support 54 on which the growing materials aredeposited is mounted on a table of the substrate holder 50 in theinterior chamber 21 which is now facing down. This configuration ischaracterized by fewer problems with particulate contamination, at thecost of a more complex wafer handling system and design of the substrateholder 50. As noted above, the deposition chamber 20 may be equippedwith optional coils or permanent magnets which may help in shaping theplasma, and is similarly equipped with effusion cells 40, etc.

Referring now to FIG. 5, another embodiment of the invention is shown,whereby the substrate supports 54 mounted on deposition assembly 50inside the chamber 20 are again facing down. The deposition chamber 20may be equipped with optional coils or permanent magnets which may helpin shaping the plasma (see FIG. 3).

In this embodiment, the elemental metal vapors are supplied to theplasma by means of water cooled sputter sources 60, holding sputtertargets 62. It is advisable to arrange the sputter targets 60 in theform of concentric rings or ring segments around the dielectric window28 of the ICP source. The sputter targets are connected through animpedance match box 64 to an RF power supply 66, whereby power supply 66provides an alternate voltage at a frequency preferably substantiallydifferent to that used by generator 34 to power the ICP coils 30. Thisreduces undesirable interferences between the two kinds of power sources34 and 66. In another embodiment of the invention, the sputter sources60 are powered by a DC power supply. It has been shown that for typicalpressures-distance products on the order of 0.2×10⁻² mbar m thethermalization of sputtered particles reaching the substrate is nearlycomplete, such that electronic-grade semiconductor material can be grownby using sputter sources (see, for example, Sutter et al., Appl. Phys.Lett. 67, 3954 (1995), the content of which is incorporated herein byreference).

In order to allow cleaning of sputter sources 60 prior to epitaxiallayer deposition, chamber 20 may be optionally equipped with a movableshutter assembly 82 allowing the shutter blade 80 to be positioned closeto and below the substrates 54 and hence avoiding any sputteredparticles to reach the substrate during pre-sputtering.

In a preferred embodiment of the invention no reactive gases are usedfor epitaxial nitride semiconductor growth at all. Additional sputtertargets 60 a may contain those doping species which are preferably usedin elemental form, such as Mg, Zn and similar metals acting as acceptorimpurities. Similarly, dopants acting as donors, such as silicon, may beprovided by additional sputter targets 60 a. In another embodiment ofthe invention each sputter gun 62 may be equipped with optional shutters(not shown) in order to avoid cross-contamination between the individualtargets 60.

During epitaxial deposition, the radio frequency power applied to theinduction coils 30 and the gas pressures in the chamber 20 are chosensuch that the heated substrates 54 are fully exposed to a low-energyplasma. Typically, gas pressures in chamber 20 range between 10⁻³ mbarto 10⁻¹ mbar, with pressures in the range of 10⁻² to 10⁻¹ mbar being themost typical. Under such conditions, activated nitrogen and metal vaporfrom sputter guns 62 both move by diffusive transport in the plasma andthe process proceeds as noted above.

In another embodiment of the invention sputter guns 62 may be combinedwith effusion cells 40, whereby both sources are preferably arrangedsymmetrically around the dielectric window 28. The combination ofeffusion cells and sputter guns for evaporating reactants and dopants inelemental form with a dense low-energy plasma suitable for epitaxiallayer deposition has not been proposed before. In a preferred embodimentof the invention more than a single sputter gun 62 and effusion cell 40are used per evaporated metal. Each source can be operated in such a wayas to deliver a different flux of metal vapors, thereby easily allowingrapid changes in growth rates or doping densities by switching from onesource to another. In still another embodiment, the effusion cells 40and sputter guns 62 may be replaced or complemented by electron beamevaporators. Electron beam evaporators are especially suitable forevaporating elements with low vapor pressures, where significant fluxesare difficult to achieve with effusion cells 40.

Referring now to FIG. 6, another embodiment of the invention is shown,in which the apparatus 10 includes a broad area plasma source 100 withan assembly of thermionic cathodes 130, an inert gas inlet 120, and anintegrated or separate anode 110. Preferably, the voltage differencebetween the cathodes 130 and the anode 110 is less than 30 V, to providethat ions striking the substrate have energy less than about 20 V. Theplasma source 100 in which an arc plasma 140 can be ignited is attachedto a deposition chamber 200. The deposition chamber, equipped with aload-lock 220, is pumped for example by a turbomolecular pump 210communicating with chamber 200 by means of valve 205, and contains asubstrate heater assembly 230. Gas lines 240 for injecting an inert gassuch as nitrogen, and additional gases, such as hydrogen, are connectedto the deposition chamber. The plasma density may be changed rapidly bychanging the confining magnetic field produced by coils 250.

In addition, this chamber is equipped with effusion cells 300 from whichmetals can be vaporized, such as Ga, In and Al. Additional cells 300 maycontain those doping species which are preferably used in elementalform, such as Mg, Zn and similar metals acting as acceptor impurities.The effusion cells are equipped with shutters 310 permitting completeinterruption of the metal vapor.

The heated assembly of substrates 400 is fully exposed to the low-energyplasma generated by the arc discharge in the plasma source and expandinginto the deposition chamber through the permeable anode 110. The arcdischarge is sustained by thermionic cathodes 130 in the plasma chamber200, and can be operated in a wide pressure range in the depositionchamber from 10⁻⁴ mbar to at least 10⁻¹ mbar, with pressures in therange of 10⁻² mbar being the most typical. Plasma activated nitrogenflowing through the deposition chamber reacts with the metal vapor,forming an epitaxial nitride film on the substrate 400.

Effusion cells are normally used for evaporating metals in ultra-highvacuum for example in a molecular beam epitaxy system. Here, they serveto introduce a metal vapor into a high-density low-energy plasmagenerated at typical pressures of about 10⁻² mbar at which transport isdiffusive. LEPEVPE is hence a process being operated under completelydifferent conditions with respect to other processes. In one embodimentof the invention, the region of the effusion cells 300 is differentiallypumped 320 in order to exclude thermal reactions with the hot metalsinside and diffusive transport in the connecting tube to the depositionchamber.

In a preferred embodiment of the invention more than a single effusioncell 300 is used per evaporated metal. Each cell can be operated at adifferent temperature, thereby easily allowing rapid changes in growthrates or doping densities by switching from one cell to another. Inaddition, changes of the plasma density, brought about by changing themagnetic field produced by the coils 250, can further enhance thedynamic range of growth rates.

In another embodiment of the invention, additional gas lines 240 a areused to insert doping gases into the deposition chamber for those dopingelements which are preferably applied in gaseous form. The doping gases,such as Silane for n-type doping, are preferably diluted in anon-reactive gas, such as argon. The dynamic range of doping can beincreased by using more than one gas line per doping gas.

The process of the invention is carbon-free because it does not requireany carbon-containing precursor gases. In a preferred embodiment, it isalso operated hydrogen-free. This embodiment is especially desirable forp-doped GaN layers since a hydrogen-free process does not need anydopant activation by thermal annealing.

Since LEPEVPE is a plasma-activated process it can be operated at lowersubstrate temperatures than competing techniques where tensile stressinduced by different thermal expansion coefficients of epilayer andsubstrate often lead to undesirable crack formation during cooling fromthe growth temperature.

ANNEX A—the below documents are incorporated herein by reference theretoand relied upon.

US patent documents 6,472,300 October 2002 Nikolaev et al. 6,706,119March 2004 Tsvetkov et al. 6,818,061 November 2004 Peczalski et al.5,633,192 May 1997 Moustakas et al. 6,454,855 September 2002 von Känelet al. 6,918,352 July 2005 von Känel et al. 5,580,420 December 1996Watanabe et al. 4,368,092 January 1983 Steinberg et al. 4,948,458 August1990 Ogle 6,811,611 November 2004 Johnson 5,788,799 August 1998 Stegeret al.

Other patent documents WO 2006/000846 January 2006 von Känel et al.

Additional publications.

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What is claimed is:
 1. A vacuum system for semiconductor epitaxy, thesystem comprising: a. a pressure-controlled deposition chamber thatmaintains pressure above 10^−3 mbar up to about 1 mbar during epitaxialdeposition; b. a substrate holder within the deposition chamber wherebya substrate is removably attached; c. sources that vaporizes substancesinto vapor particles and discharge the same to the deposition chamber,such vapor particles including elemental metals, metal alloys anddopants; d. a gas supply system that supplies gases to the depositionchamber; and e. a plasma source that supplies a suitable plasma to thedeposition chamber, the discharge of the sources being disposed withrespect to the substrate holder so as to enable direct contact with thesubstrate; wherein the system is adapted to propagate gas and vaporparticles diffusively in the deposition chamber, the gas and particlesbeing activated by the plasma, so as to react and form a uniformepitaxial layer on a heated substrate fixed to the substrate holder bylow-energy plasma enhanced vapor phase epitaxy.
 2. The system of claim1, wherein the plasma source is selected from a group consisting ofinductively coupled plasma (ICP) sources and low-voltage arc dischargesources.
 3. The system of claim 1, wherein the sources for vaporizingsubstances supplying elemental metal and metal alloy vapors are chosenfrom a group of sources capable of delivering vapor fluxes.
 4. Thesystem of claim 1, wherein the dopant sources are chosen from a group ofsources consisting of effusion cells, splitter sources and reactive gassources.
 5. The system of claim 1, wherein the gases supplied to thedeposition region by the gas distribution system are non-toxic,non-reactive gases.
 6. The system of claim 2, wherein the ICP sourceincludes a spiral inductive coupler.
 7. The system of claim 2, whereinthe plasma source is a direct-current plasma source containing one ormore thermionic cathode; and wherein the assembly of cathodes in theplasma source communicates with an anode permeable to the plasma.
 8. Thesystem of claim 7, wherein the direct current plasma source is abroad-area source whereby the region of plasma extraction is formed by ashower head, and whereby said shower head communicates with thepermeable anode that forms a uniform plasma density in the depositionregion.
 9. The system of claim 1, wherein the gases activated by theplasma are allowed to react in the deposition region with plasmaactivated metal vapors, thereby forming an epitaxial template layer, onheated substrates fixed to the heated substrate holder, which templatelayer is of a quality sufficient for subsequent epitaxy of nitridesemiconductor layers.
 10. The system of claim 1, wherein nitrogen gas issupplied to the deposition region by the gas distribution system andactivated by the plasma and allowed to react in the deposition regionwith plasma activated metal vapors chosen among a group of metalscomprising Ga, In, and Al, thereby forming an epitaxial nitridesemiconductor layer on substrates held by the heated substrate holder.11. The system of claim 1, wherein the dopant sources contain elementschosen among a group of elements, comprising Si, Mg, Zn, Be, and Cd. 12.The system of claim 1, further equipped with shutters permittinginterruption of a flux of the metal and dopant vapors.
 13. The system ofclaim 1, wherein at least one of the sources is differentially pumped.14. The system of claim 1, wherein more than one source is provided foruse with each evaporated substance, and wherein the sources used for thesame substance are adapted to yield different fluxes of vapor particlesin operation.
 15. The system of claim 4, wherein the system is adaptedto feed doping gases into the deposition chamber.
 16. The system ofclaim 2, wherein the plasma source is a low voltage arc discharge sourcewherein the arc voltage is below 30 V, wherein ions striking thesubstrate have energy less than about 20 V.
 17. The system of claim 1,further comprising a magnetic field generator that changes the magneticfield strength at the substrate position enabling rapid changing of theplasma density at the substrate.
 18. The system of claim 1, furthercomprising a magnetic field generator that periodically changes thedirection of the magnetic field.
 19. The system of claim 3, wherein thesources for vaporizing substances supplying elemental metal and metalalloy vapors include effusion cells.
 20. The system of claim 3, whereinthe sources for vaporizing substances supplying elemental metal andmetal alloy vapors include sputter sources.
 21. The system of claim 5,wherein the gas supplied to the deposition region by the gasdistribution system is argon.
 22. The system of claim 5, wherein the gassupplied to the deposition region by the gas distribution system isnitrogen.
 23. The system of claim 15, wherein the doping gases aresilane diluted with an inert gas.